Who are ISIS-K, the group suspected of being behind the attack near Kabul airport?

US officials strongly believe the ISIS-Khorasan group was behind the attack on Thursday at Kabul's airport, a source familiar with congressional briefings on Afghanistan said on Thursday. A second US government source familiar with intelligence activities said that while the US government is still investigating, the airport attack has "all the hallmarks" of an ISIS-K attack. Who are the ISIS-Khorasan group?